1. Field of the Invention
The present invention relates to surface acoustic wave devices and, more particularly, to a surface acoustic wave device having a balanced-to-unbalanced conversion function, and to a communication device incorporating the surface acoustic wave device.
2. Description of the Related Art
In recent years, cellular phones have become remarkably smaller and lighter. In order to realize such smaller and lighter cellular phones, a smaller size of cellular phone components and components in which a plurality of functions are integrated have been developed. Against a background of such circumstances, research has been extensively performed on a surface acoustic wave filter having a balanced-to-unbalanced conversion function, which is commonly called a balun, used in the RF stage of a cellular phone, and these have begun to be put into practical use. Several patent applications for such surface acoustic wave filters having a balanced-to-unbalanced conversion function (hereinafter referred to as “surface acoustic wave filters with a balanced-to-unbalanced conversion function”) have been filed.
Depending on the system configuration of a cellular phone, there are cases in which a surface acoustic wave filter with a balanced-to-unbalanced conversion function, having an impedance of a balanced terminal which is approximately four times as high as the impedance of an unbalanced terminal, is required. Examples of surface acoustic wave filters meeting such a demand include a surface acoustic wave filter, in which the terminals of IDTs which are connected in series (hereinafter referred to simply as “series IDTs”), disposed along a direction that is perpendicular to the propagation direction of surface acoustic waves, are formed as balanced terminals. For example, in an unexamined patent publication “Japanese Unexamined Patent Application Publication No. 2001-292050 (Publication Date: Oct. 19, 2001)”, a surface acoustic wave filter in which terminals of series IDTs are formed as balanced terminals is disclosed.
FIG. 14 shows a surface acoustic wave filter 100 as a conventional example in which terminals of series IDTs are formed as balanced terminals. In FIG. 14, a conductor thin-film pattern formed on a piezoelectric substrate (not shown) is indicated by hatching.
In the surface acoustic wave filter 100, surface acoustic wave reflectors 101 and 102 formed such that a plurality of conductor thin-film fine lines are arranged in parallel are disposed with a spacing therebetween, and IDTs (Interdigital Transducers) 103, 104, 105, and 106 are arranged between the surface acoustic wave reflectors 101 and 102.
The IDT 103 is disposed adjacent to the surface acoustic wave reflector 101, and the IDT 106 is disposed adjacent to the surface acoustic wave reflector 102. One of the terminals of the IDT 103 is grounded, and the other terminal is connected to an unbalanced terminal 107. In a similar manner, one of the terminals of the IDT 106 is grounded, and the other terminal is connected to the unbalanced terminal 107.
The IDT 104 and the IDT 105 are arranged so as to be sandwiched between the IDT 103 and the IDT 106. Furthermore, the IDTs 104 and 105 are arranged so as to be arranged to define upper and lower portions so as to approximately divide the width of the propagation path of the surface acoustic waves into two portions. The adjacent terminals of the IDTs 104 and 105 are integrally provided as a common electrode 108. Furthermore, the terminals on the side which is not of the common electrode 108 of the IDT 104 are connected to a balanced terminal 109. In a similar manner, the terminal on the side which is not of the common electrode 108 of the IDT 105 is a balanced terminal 110. That is, the IDT 104 and the IDT 105 are series IDTs, and the common electrode 108 is a series connection point thereof.
In the surface acoustic wave filter 100, when an alternating signal is input to the unbalanced terminal 107, surface acoustic waves are emitted from the IDT 103 and the IDT 106, thereby generating standing surface acoustic waves between the surface acoustic wave reflector 101 and the surface acoustic wave reflector 102. Next, the IDTs 104 and 105 absorb the surface acoustic waves from these waves, and generates an electrical signal. Since each of the IDTs 104 and 105 converts approximately half of the surface acoustic waves in the vertical direction into electrical signals, the amplitudes of the electrical signals generated by them nearly match each other. Furthermore, since the polarity of the balanced terminal 109 connected to the IDT 104 is opposite to the polarity of the balanced terminal 110 to which the IDT 105 is connected, the phases of the electrical signals generated in them differ by approximately 180°.
Based on the above-described operating principles, in the surface acoustic wave filter 100, when an alternating signal is input from the unbalanced terminal 107, a balanced output having an approximately opposite phase and the same amplitude is output from the balanced terminal 109 and the balanced terminal 110. Furthermore, since the efficiency of the electrical signal-to-surface acoustic wave conversion, performed in the IDTs 103 to 106, has a frequency characteristic, a frequency filter characteristic is obtained between the unbalanced terminal 107 and the balanced terminals 109 and 110. This is the same characteristic as that seen in general surface acoustic wave devices.
The surface acoustic wave filter 100 performs an operation which is opposite to the foregoing when the unbalanced terminal 107 is an output terminal, and the balanced terminals 109 and 110 are input terminals.
Here, in the surface acoustic wave filter with a balanced-to-unbalanced conversion function, the degree of balance, that is, when an alternating signal is input to unbalanced terminals, by how much the amplitudes of the alternating signals output from the two balanced terminals are aligned (hereinafter referred to as a “degree of amplitude balance”), and by how much the difference of the phases of the alternating signals output from the two balanced terminals is close to 180° (hereinafter referred to as a “degree of phase balance”), become important characteristic indicators.
However, the surface acoustic wave filter with a balanced-to-unbalanced conversion function of the above-described conventional configuration has a problem in that the degree of balance is not sufficient for the surface acoustic wave filter to be incorporated in a cellular phone, and therefore, further improvements are needed.
In this respect, as one of the reasons that the degree of balance cannot be obtained sufficiently, noise which comes from an electrical-potential signal generated at the series connection point of series IDTs can be given. Specifically, in an ideal operating state, an electrical-potential signal will not be generated at the series connection point of series IDTs, but in practice, several electrical-potential signals are generated. Thus, the electrical-potential signals which are generated at the series connection point cause noise to occur in the two balanced terminals via a parasitic capacitor, etc. At this time, the phase of the noise generated in one of the balanced terminals and the phase of the noise generated in the other balanced terminal are not always in an inverted relationship. As a result, this noise causes the degree of balance of the balanced terminals to be worsened.